Physical mechanisms of hypoxic events and temperature inversions in Bahia Almirante, a shallow tropical embayment in Bocas del Toro, Panama

Anne Elizabeth Adelson, Scripps Institution of Oceanography, La Jolla, United States, Kristen A Davis, University of California Irvine, Civil and Environmental Engineering Department, Irvine, CA, United States, Sarah N Giddings, Scripps Institution of Oceanography, La Jolla, CA, United States, Rachel Collin, Smithsonian Tropical Research Institute, Calzada de Amador, Panama, Victoria Reed, McGill University, QC, Canada, Andrew Altieri, University of Florida, Department of Environmental Engineering Sciences, Gainesville, United States and Geno R Pawlak, University of California San Diego, Mechanical and Aerospace Engineering, La Jolla, CA, United States
Abstract:
Hypoxia is a prevalent phenomenon throughout the world’s oceans, with complex impacts on marine ecosystems. Coastal oxygen depletion has primarily been studied in temperate waters, and research suggests that the occurrence of hypoxia-related dead zones in the tropics is underreported. We examine the physical processes that are important in regulating hypoxia in Bahia Almirante, a semi-enclosed tropical embayment on the Caribbean coast of Panama, which has recently experienced widespread hypoxia-induced coral mortality.

Long-term monitoring observations conducted by the Smithsonian Tropical Research Institute reveal seasonal variations in dissolved oxygen (DO) concentrations, with intermittent occurrences of hypoxia at depth. We found that hypoxic events are more severe and persistent towards the back of the bay, though they occur throughout Bahia Almirante. Reduction in DO corresponds with high freshwater inputs from rivers and direct precipitation, resulting in strong salinity stratification that isolates bottom waters. Advection and vertical mixing contribute to intermittent breakdown of events throughout the bay, with bay-wide breakdown of hypoxia coinciding with the end of the rainy season. In order to compare various mechanisms, non-dimensional stability parameters, which take the form of a Richardson number, were used to examine the relative contributions of wind-driven mixing, convective mixing, and shear from estuarine exchange.

Temperature inversions also exhibit a seasonal pattern in which bottom temperatures are frequently warmer than surface temperatures in July/August and November/December. This coincides with the onset and breakdown of hypoxic conditions, respectively. Temperature inversions and hypoxia tend to follow similar spatiotemporal patterns, suggesting these phenomena are related and modulated by the same environmental drivers. Our findings reveal that coastal hypoxia in tropical ecosystems exhibits seasonal patterns related to climatological regime.

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